From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 5075B3A76411; Wed, 28 Apr 2021 08:43:43 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 5075B3A76411 From: "stefansf at linux dot ibm.com" To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/100263] [11/12 Regression] RTL optimizers miscompile loop Date: Wed, 28 Apr 2021 08:43:43 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: rtl-optimization X-Bugzilla-Version: 11.0 X-Bugzilla-Keywords: wrong-code X-Bugzilla-Severity: normal X-Bugzilla-Who: stefansf at linux dot ibm.com X-Bugzilla-Status: NEW X-Bugzilla-Resolution: X-Bugzilla-Priority: P2 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: 11.2 X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 28 Apr 2021 08:43:43 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100263 --- Comment #6 from Stefan Schulze Frielinghaus --- Prior postreload we have (insn 12 379 332 3 (set (reg:QI 17 %f2 [orig:198 l_lsm_flag.27 ] [198]) (const_int 1 [0x1])) 1480 {*movqi} (expr_list:REG_EQUIV (const_int 1 [0x1]) (nil))) which gets substituted during postreload by (insn 12 379 332 3 (set (reg:QI 17 %f2 [orig:198 l_lsm_flag.27 ] [198]) (reg:QI 17 %f2 [orig:198 l_lsm_flag.27 ] [198])) 1480 {*movqi} (expr_list:REG_EQUIV (const_int 1 [0x1]) (nil))) which gets deleted during split2 where we have deleting insn with uid =3D 12. The culprit seems to be that postreload changes the RHS of the assignment to `reg:QI 17 %f2` which is wrong. Register %f2 holds indeed constant 1, howev= er, in DImode which is not compatible to QImode on IBM Z. The decision takes place in function move2add_valid_value_p where the wrong offset is returned by call subreg_regno_offset (17, DImode, 7, QImode) I would have expected offset 7 but 0 is returned which renders the subseque= nt if-condition false. The 0 comes from function call subreg_get_info which returns in this case via /* Lowpart subregs are otherwise valid. */ if (!rknown && known_eq (offset, subreg_lowpart_offset (ymode, xmode))) { info->representable_p =3D true; rknown =3D true; if (known_eq (offset, 0U) || nregs_xmode =3D=3D nregs_ymode) { info->offset =3D 0; info->nregs =3D nregs_ymode; return; } } The offset doesn't equal zero but the number of registers. Still the offset= is set to zero. I did a quick test by using instead diff --git a/gcc/postreload.c b/gcc/postreload.c index dc67643384d..64297be2c45 100644 --- a/gcc/postreload.c +++ b/gcc/postreload.c @@ -1732,12 +1732,7 @@ move2add_valid_value_p (int regno, scalar_int_mode m= ode) (REG:reg_mode[regno] regno). Now, for big endian, the starting regno of the lowpart might be different. */ poly_int64 s_off =3D subreg_lowpart_offset (mode, old_mode); - s_off =3D subreg_regno_offset (regno, old_mode, s_off, mode); - if (maybe_ne (s_off, 0)) - /* We could in principle adjust regno, check reg_mode[regno] to be - BLKmode, and return s_off to the caller (vs. -1 for failure), - but we currently have no callers that could make use of this - information. */ + if (simplify_subreg_regno (regno, old_mode, s_off, mode) < 0) return false; } which works at least for the example (haven't done a bootstrap nor regtest yet). However, I'm still wondering whether subreg_get_info is supposed to return with a zero offset in cases like this? Any thoughts?=